Mr Ryan also joined other military figures in calling for Prime Minister David Cameron to cancel his second holiday of the summer and recall parliament to deal with the situation in Iraq.

Former head of the British Army, General Sir Richard Dannatt, claimed the "nation would expect" MPs to return to Westminster over the ongoing crisis.

The ex-defence adviser to Mr Cameron told BBC Breakfast: "I think Members of Parliament and members of the House of Lords will get very frustrated if they stay in recess through August and September until October.

"It is not the same as last year, the end of August, when there was a specific proposition that the British might support the Americans in bombing [Syrian president Bashar] Assad's chemical capability, that was a specific issue.

"But I think there will come a point as this general set of circumstances unfolds, when Parliament needs to come back together, people need to have a full debate about it and express their point of view.